1. Field of the Invention
This invention relates to an oxygen concentration sensor abnormality-detecting system for internal combustion engines, which detects abnormality of an oxygen concentration sensor arranged in the exhaust system of the engine at a location upstream of a catalytic converter arranged therein.
2. Prior Art
To detect abnormality of an oxygen concentration sensor arranged in the exhaust system of an internal combustion engine at a location upstream of a catalytic converter arranged therein, an oxygen concentration sensor abnormality-detecting system has been proposed by Japanese Utility Model Publication (Kokoku) No. 62-28675, which determines that the oxygen concentration sensor is abnormal if the output level of the oxygen concentration sensor remains less than a predetermined value, i.e. stays on a lean side when an increased amount of fuel is supplied to the engine.
According to this abnormality-detecting system, however, the detection of abnormality of the oxygen concentration sensor has to be carried out only when the amount of fuel supplied to the engine is increased. On the other hand, from the standpoint of fuel economy, it is required that the frequency of increase of the fuel amount supplied to the engine should be as low as possible. Consequently, the detection of abnormality of the oxygen concentration sensor cannot be carried out so long as no increase of the fuel amount supplied to the engine is required, resulting in a very low frequency of execution of the abnormality detection.
Further, a gas engine in particular undergoes large variations in the air-fuel ratio of a mixture supplied to the engine due to variations in the composition of the fuel such that the air-fuel ratio of the mixture cannot be surely enriched even when the fuel supply amount is controlled to an increased amount, which can result in an erroneous detection of abnormality of the oxygen concentration sensor.